The present invention relates to 2-debenzoyl-4-deacetyl paclitaxel, 2-debenzoyl-4-deacetyl-2,4-diacyl paclitaxel analogs thereof, and methods for making the same.
The natural product paclitaxel (1) (Taxol(copyright)) is an effective antitumor drug with demonstrated clinical activity against breast and ovarian cancer, as well as indicated activity against non-small cell lung cancer (24, 25). Studies of use against various other cancers show promising results. Recent studies have elucidated the unique mode of action of paclitaxel, which involves abnormal polymerization of tubulin and disruption of mitosis. Taxol was first isolated and its structure reported by Wani, et al. (26). 
Taxol is found in the stem bark of the western yew, Taxus brevifolia, as well as in T. baccata and T. cuspidata. Therefore, there is a limited natural supply of paclitaxel.
Because of the limited availability of paclitaxel and the high demand due to its efficacy against various types of cancer, other derivatives and analogs of paclitaxel have been sought. The relative scarcity of such analogs in relation to their importance as potential anti-cancer agents is due to several factors, including the large size and complexity of paclitaxel compounds, the presence of multiple reactive sites and the presence of many stereospecific sites, making synthesis of even close analogs difficult.
Because it is believed that the tetracyclic taxane nucleus is an important feature in establishing the antineoplastic activity of paclitaxel and analogs thereof, it is desired to alter the ring substituents without disrupting the tetracyclic nucleus in order to develop antineoplastically active derivatives of paclitaxel. The complexity of paclitaxel and its analogs makes it difficult to selectively alter substituents.
The only disclosed preparations of taxol analogs retaining the tetracyclic taxane nucleus are those analogs modified at the C-1, C-2, C-4, C-7 and C-13 positions, and derivatives having a protecting group or a hydroxyl group at the C-10 position (27). However, it has been demonstrated that analogs with improved activity can be prepared by modifications at various functional groups, and several investigators have prepared paclitaxel analogs modified at the 2-position (1-10), at the 4-position (11-15), at the 7-position (16-19), at the 9 and 10 positions (20-21and at the 14-position (22), among others. Baccatin III derivatives (baccatin III is the taxane core of paclitaxel) have also been prepared with substitutions at C-2 and/or C-4 (23).
In particular, analogs at the C-2 position have been prepared by Chaudhary et al. (1,3) using a phase-transfer catalyst to prepare 2-debenzoylpaclitaxel followed by reacylation with a carboxylic acid in the presence of dicyclhexylcarbodiimide (DCC) and pyrollidinopyridine (PP). Similar chemistry has more recently been reported by Georg et al. (2,5,6,13), who used potassium t-butoxide as the base and reacylated in the presence of 1,3-dicyclohexylcarbodiimide (DCC) and N,N-dimethylaminopyridine (DMAP). Nicolaou (4,7) has shown that 2-debenzoylpaclitaxels can be prepared from 10-deacetylbaccatin III by a process involving protection at C-7, oxidation at C-13, selective debenzoylation at C-2, formation of the cyclic 1,2-carbonate derivative, reaction with an aryllithium, reduction at C-13, and finally coupling of the C-13 side chain. A different route to 2-debenzoyl taxoids was developed by Pulicani et al. (8), who were able to prepare 2-debenzoyl docetaxel and certain derivatives by electrochemical reduction of docetaxel followed by reacylation with Butyllithium and an acid chloride. Yet another, albeit restricted, synthesis of certain 2-acyl paclitaxel analogs was achieved by Ojima et al. (9) and Boge et al. (10), who independently hydrogenated baccatin III to its 2-cyclohexylcarbonyl derivative, and then attached the C-13 ester side chain. 2-Debenzoylbaccatin III was prepared by Datta et al. (23) by treatment of 7,13-bis(triethylsilyl)baccatin m with potassium t-butoxide.
4-Deacetylpaclitaxel has been prepared by Neidigh et al. (11) and independently by Georg et al. (12). Neidigh et al. prepared 4-deacetylpaclitaxel by treatment of a protected paclitaxel with base under various conditions, and also by a second method in which a C-13 side chain was attached to a suitably protected 4-deacetylbaccatin III. Georg et al. prepared 4-deacetylpaclitaxel by attachment of the C-13 side chain to a protected deacetylbaccatin III. 4-Deacetylbaccatin III was also prepared by Datta et al. (23) by treatment of 7-(triethylsilyl)baccatin III with potassium t-butoxide. C-4 deacetoxypaclitaxel was prepared by Chordia et al. (15) by preparation of 2-debenzoyl-4-deacetyl paclitaxel by treatment of 2-t-butyldimethylsilyl-7-triethylsilylpaclitaxel with Triton B (an organic-soluble base) followed by formation of the cyclic 1,2-carbonate, formation of a xanthate at C-4, opening of the carbonate with phenyllithium, and deprotection.
Paclitaxels with modified C-4 acyl substituents have been prepared by Chen et al. (14), who protected 7,13-di(triethylsilyl)baccatin III at C-1 with a dimethylsilyl protecting group and then deacylated selectively at C-4 with Red-Al. Subsequent reacylation using acid chloride and lithium hexamethyldisilazide (LHDMS), followed by protecting group manipulations and reacylation at C-13 with the paclitaxel side chain (as its xcex2-lactam derivative) yielded a range of 4-acylpaclitaxel analogs. A 4-acyl analog of paclitaxel was also prepared by Georg et al. (13), who treated 2xe2x80x2-t-butyldimethylsilyl-7-triethylsilylpaclitaxel with aqueous potassium t-butoxide to give a 2-debenzoyl-4-deacetyl-2xe2x80x2-t-butyldimethylsilyl-7-triethylsilylpaclitaxel. This compound was converted to its cyclic carbonate, acylated at C-4, and treated with phenyllithium to yield a 4-isobutyroylpaclitaxel analog.
In spite of all the work that has been done on the preparation of paclitaxel analog with C-2 and C-4 acyl substituents, no work has been reported to date on the preparation of derivatives with modified substituents at both C-2 and C-4. The preparation of such derivatives is desirable because it is anticipated that such derivatives will have antineoplastic activity, like paclitaxel itself. Further, because previous work has shown that both 2-acyl and 4-acyl analogs independently can have improved activity over paclitaxel, it is thought that some derivatives described herein will also have improved activity. It is further contemplated that the derivatives described herein will be easier to synthesize, will be more abundant, will have greater solubility and/or will have fewer side effects than paclitaxel.
The preparation of analogs of paclitaxel is an important endeavor, especially in view of paclitaxel""s clinical activity and limited supply. The preparation of analogs might result in the synthesis of compounds with greater potency than paclitaxel (thus reducing the need for the drug), compounds with superior bioavailability, or compounds which are easier to synthesize than paclitaxel from readily available sources.
The present application describes paclitaxel analogs which have modified substituents at both the C-2 and C-4 positions, in particular 2-debenzoyl-2-acyl-4-deacetyl4-acyl paclitaxel analogs, as well as pro compounds, and intermediates which can be utilized in preparing these compounds.
The compounds of the present invention have anti-neoplastic activity and may be used to treat patients suffering from cancer, or as intermediates for making compounds which can be used to treat cancer. In a preferred embodiment, the paclitaxel analogs have improved in vivo activities for use as anticancer agents, are more soluble, and/or have fewer side effects than paclitaxel.
Compounds of the present invention include compounds having the general formula: 
wherein R1 is an aryl or substituted aryl; R2 is an aryl or substituted aryl; R3 is selected from the group consisting of H, OH, and OC(O)Ra; R4 is selected from the group consisting of H, OH, oxyprotecting group (i.e. triethylsiloxy), ORb, and OC(O)Rc, and wherein Ra, Rb, and Rc are independently selected from the group consisting of alkyls, aryls, and substituted aryls; R5 is selected from the group OH, OC(O)Rd, OC(O)ORe and OC(S)SRf; and R6 is selected from the group H and OC(O)Rg, where Rd, Re, Rf and Rg are independently selected from the group consisting of alkyls, cycloalkyls, heterocycloalkyls, heterocycloaryls, alkenyls, alkynyls, aryls, and substituted aryls. As used herein, substituted aryl means an aryl independently substituted with one to five (but preferably one to three) groups selected from C1-6 alkanoyloxy, hydroxy, halogen, C1-6 alkyl, trifluoromethyl, C1-6 alkoxy, aryl, heteroaryl, C2-6 alkenyl, C1-6 alkanoyl, nitro, amino, cyano, azido, C1-6 alkylamino, di-C1-6 alkylamino, and amido.
Preferred embodiments of the present invention include compounds having the formula: 
wherein Ar is a phenyl or substituted phenyl group and R5 is an alkyl, cycloalkyl or an alkoxy group. Another preferred embodiment includes compounds having the formula: 
wherein Ar is a substituted phenyl and R7 is an alkyl, substituted alkyl, aryl or substituted aryl group. As used herein, a substituted phenyl group may be alkyl, alkenyl, alkynyl, aryl, heteroaryl and/or may contain nitrogen, oxygen, sulfur, halogens and include, for example, lower alkoxy such as methoxy, ethoxy, butoxy; halogen such as chloro or fluoro; nitro; amino; and keto.